Engineers working for the Assad regime in Syria have begun combining the two chemical precursors needed to weaponize sarin gas, an American official with knowledge of the situation tells Danger Room. International observers are now more worried than they’ve even been that the Damascus government could use its nerve agent stockpile to slaughter its own people.

The U.S. doesn’t know why the Syrian military made the move, which began in the middle of last week and is taking place in central Syria. Nor are they sure why the Assad government is transferring some weapons to different locations within the country, as the New York Times reported on Monday.